To measure particle or quantum radiation, especially that of low-energy particles, liquid isotopes called liquid scintillators are added to the sample, which may be liquid or solid, to convert the particles or quanta into a photon beam of proportional energy. The samples are introduced along with the liquid scintillator into a transparent container of glass or plastic, for example. The photons travel through the walls of the container, and their radiation is detected by light-sensitive detectors. The light-sensitive detectors employed in known instruments are photomultipliers which can measure individual photons. In order to eliminate cathode noise, the photomultipliers are employed in a coincident mode, that is, two photomultipliers are positioned against the sample container where they can count only the particle beams that simultaneously supply a signal to both multipliers with a period of coincidence, usually 20 nanoseconds. The counting rate arriving from the detectors is calculated by electronic data processing and subtracted from the overall signal, or detected and eliminated by special logic circuits.
To obtain sufficient precision from these tests they are usually maintained for between one and ten minutes. In practice, many more than one sample is involved, and systems with as many as six pairs of detectors are known. Providing the instruments with an automatic sample replacer is also known. Automatic sample replacers that can handle up to 500 samples, which must, however, be measured sequentially for the appropriate length of time, are already being employed. The lengthiness of the known procedures is a drawback for many reasons. A sample replacer that can handle 500 samples at as little as 1 minute per test takes more than 8 hours to complete the series. When a test takes 10 minutes, it takes 80 hours to process 500 samples. Since the samples can change in many ways over time, another drawback is that the results will no longer represent the original conditions.
Also known are what are called automatic sample-preparation systems, wherein the samples are automatically pipetted into standard microplates with 96 wells each. This approach takes 1.6 to 16 hours.